1. Field of the Invention
This invention relates to a photoelectric oscillator capable of acquiring energy from an incident laser beam and generating a millimeter wave, microwave or other radio-frequency signal, particularly a photoelectric oscillator whose internal optical modulator is provided with a frequency selection characteristic to enable generation of a signal having a high frequency selected by the selection characteristic or a signal having a frequency that is a natural number multiple of the high frequency selected by the selection characteristic.
2. Description of the Prior Art
The photoelectric oscillator is an oscillator that achieves oscillating operation by feeding back a sideband component produced by laser beam modulation as a modulation signal. The feedback loop of the oscillator is constituted of an optical circuit and an RF (radio-frequency) circuit. During oscillating operation, therefore, it enables, without use of an external modulation signal, simultaneous acquisition of a signal composed of an oscillation signal of the same frequency superimposed on a modulated signal and an RF signal. It is also well known that injection-locked operation of the photoelectric oscillator can be achieved by inputting an optical signal or an RF signal, thereby enabling oscillation frequency control.
A block diagram of the structure of a conventional photoelectric oscillator is shown in FIG. 1. A laser beam from a laser beam source is intensity-modulated by an optical modulator, the intensity-modulated beam is amplified, and the amplified bcam is converted to a radio-frequency signal by a photodiode. The radio-frequency signal is amplified, passed through a bandpass filter, and again used to modulate the laser beam.
Thus the conventional photoelectric oscillator suppresses loop gain outside the desired frequency band to enable single mode oscillating operation. The suppression is usually achieved by using a bandpass filter. The conventional photoelectric oscillator therefore unavoidably has a complex configuration. Moreover, the use of a traveling-wave broadband modulator as the optical modulator increases the size of the photoelectric oscillator large and makes improvement of modulation efficiency difficult. The amount power required during oscillating operation, including that for the laser beam, is therefore disadvantageously large.
During photoelectric oscillation, an optical signal and an electric signal of the same frequency are simultaneously output from the “optical output” section and the “RF output” section (see FIG. 1). As is well known, such a photoelectric oscillator is usually configured so that the upper limit of the oscillation frequency is governed by the upper limit of the frequency band of the electric circuit in the feedback circuit section. This is because, generally, broadband operation of the optical circuit can be adequately achieved but making the electric circuit capable of broadband operation is technically difficult. Because of the limited bandwidth of the electric circuit, the upper frequency limit of the optical modulated signal obtainable with a conventional photoelectric oscillator is low.